The present invention relates to optical couplers and, more particularly, one which is made up of a number of waveguides which are substantially coplanar, each optically coupling a single coupler input to one of 2.sup.n coupler outputs. The waveguides comprise waveguide sections in a tree and branch arrangement connecting n Y-junctions disposed along each waveguide path. The invention also relates to a process for the manufacture of said coupler.
Such optical couplers are known in the art and are also called beam splitters or combiners according to whether they divide or combine light beams. They are utilized in diverse applications such as, for example, telecommunications by multimode or single mode optical fibers within a range of wavelengths from 600 to 1600 nm.
Thus, various waveguide patterns have been proposed for such couplers, which can be integrated in a substrate by various techniques which involve masking and photolithography combined with ion exchange, deposition, diffusion, etching or epitaxial growth techniques, depending upon the type of substrate used, for forming the waveguides within or upon the substrate following a design defined by the mask.
FIG. 1 depicts a prior art example of such a design which has been utilized, for example, to create a coupler with one input and sixteen outputs which is called a 1 by 16 coupler. Only one half of the waveguide pattern of the coupler has been represented, as the design is symmetric with respect to the longitudinal axis, X, of the coupler. The representation given here has been significantly distorted parallel to axis, X, in order to present the geometric characteristics more clearly.
If one assumes that the coupler is utilized as a splitter designed to receive a light beam at its input junction J11 and to divide this beam in a tree and branch manner into 2.sup.n beams (n=4) coming out at as many outputs S1 to S16 separated by constant spacing (only outputs S1 to S8 are represented) it is apparent that n stages of junctions are necessary to assure the division of the input beam, these stages of junctions being formed respectively by the junctions: J11; J21 etc.; J31, J32 etc.; and, J41, J42, J43, J44 etc.
Considering now the pattern of the waveguide sections which couple these junctions, it becomes evident that these sections exhibit multiple inflection points, as much within the junctions themselves as between those junctions where one again finds a inflection point. These multiple inflection points are necessary, along a given section, in order to return the axis of symmetry of each junction parallel to the longitudinal X axis of the coupler. Such a pattern is known in the prior art, for example, from German patent No. 25 16 975.
The required longitudinal dimension L.sub.1 of the pattern of FIG. 1 is penalized by the multiple changes of curvature of the waveguide sections which connect the junctions. It is desirable to reduce this required dimension for several reasons.
On the one hand, as couplers of the type in question are fabricated by photolithographic techniques upon a substrate by a plurality of patterns reproducing the same elementary design, a reduction of the required longitudinal dimension of the design would permit the manufacture of more couplers upon the same substrate surface, thereby increasing, the production rate for the coupler and at the same time lowering the cost of manufacture.
On the other hand, couplers produced by integrated optics take the form of elongated bars of glass or silica, for example, several centimeters in length. The greater the length of these bars, the more susceptible they are to cutting stresses which may result in shattering. A reduction of their length would reduce their fragility.
To reduce the required longitudinal dimension of the coupler one could consider displacing the design of the waveguides laterally by relinquishing the idea of maintaining junctions parallel to the longitudinal axis of the coupler as is represented in FIG. 2 of Voirin, "Performances of an Ion Exchanged Star Coupler for Multimode Optical Communications", pages 229 to 231, in "Integrated Optics - Proceedings of the 3rd European Conference, ECIO '85, BERLIN, Germany, May 6-8, 1985" published by Springer-Verlag. The design proposed by Voirin, only for a multimode coupler, contains angular points at all of the junctions, and other angular points between the last stage of junctions and the coupler outputs; these angular points or singularities, result in an estimated loss of light energy of approximately 0.2 dB across the transition. These losses can negatively affect not only the transmitted optical signal power of the coupler, but also the uniformity of the power transmitted by each of the coupler outputs.